The present invention relates to improving the performance of an organic electroluminescent (EL) device, especially relates to improving the luminous efficiency of an EL device.
Organic electroluminescent (EL) devices or organic light-emitting devices (OLEDs) are electronic devices that emit light in response to an applied potential. The structure of an OLED comprises, in sequence, an anode, an organic EL medium, and a cathode. The organic EL medium disposed between the anode and the cathode is commonly comprised of an organic hole-transporting layer (HTL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the ETL near the interface of HTL/ETL. Tang et al. demonstrated highly efficient OLEDs using such a layer structure in xe2x80x9cOrganic Electroluminescent Diodesxe2x80x9d, Applied Physics Letters, 51, 913 (1987) and in commonly assigned U.S. Pat. No. 4,769,292. Since then, numerous OLEDs with alternative layer structures have been disclosed. For example, there are three-layer OLEDs that contain an organic light-emitting layer (LEL) between the HTL and the ETL, such as that disclosed by Adachi et al., xe2x80x9cElectroluminescence in Organic Films with Three-Layer Structurexe2x80x9d, Japanese Journal of Applied Physics, 27, L269 (1988), and by Tang et al., xe2x80x9cElectroluminescence of Doped Organic Thin Filmsxe2x80x9d, Journal of Applied Physics, 65, 3610 (1989). The LEL commonly consists of a host material doped with a guest material. Further, there are other multilayer OLEDs that contain additional functional layers, such as a hole-injecting layer (HIL), and/or an electron-injecting layer (EIL), and/or an electron-blocking layer (EBL), and/or a hole-blocking layer (HBL) in the devices. At the same time, many different types of EL materials are also synthesized and used in OLEDs. These new structures and new materials have further resulted in improved device performance.
One of the ways to improve luminous efficiency is to modify hole-transporting region (HTR) in OLEDs. A conventional OLED structure is shown in FIG. 1A, wherein OLED 100 includes an anode 120, an HTR 131, a LEL 134, an ETL 138, and a cathode 140. This device is externally connected to a voltage/current source 150 through electrical conductors 160. This device only has one HTL in the HTR, and it usually cannot produce high luminous efficiency due to un-balanced carrier injection into the LEL. In order to obtain high luminous efficiency, people are trying to use more than one HTL in OLEDs. Shown in FIG. 2B is another type of OLED structure disclosed in prior art, wherein the HTR 131 of OLED 200 contains more than one HTLs, i.e. HTL 1, . . . HTL n, (n greater than 1, an integer). HTL 1 is denoted as HTL 131.1, HTL 2 is denoted as HTL 131.2, and HTL n is denoted as HTL 131.n in the Figures. Shirota et al. reported in xe2x80x9cMultilayered Organic Electroluminescent Devices Using a Novel Starburst Molecule, 4,4xe2x80x2,4xe2x80x3-Tris(3-Methylphenylphenylamino)Triphenylamine, as a Hole Transport Materialxe2x80x9d, Applied Physics Letters, 65, 807 (1994) that an OLED with dual HTLs could increase efficiency and lifetime. They achieved higher efficiency and longer lifetime when using dual HTLs, 4,4xe2x80x2,4xe2x80x3-tris(3-methylphenylphenylamino)triphenylamine (m-TDATA)/4,4xe2x80x2-bis(3-methylphenylphenylamino)biphenyl (TPD) than those when using a single TPD layer as an HTL in their device. Egusa et al. in U.S. Pat. No. 5,343,050 shows an OLED structure containing more than two hole-transporting layers. Moreover, doped HTL (or doped HIL) is used in HTR to improve the luminous efficiency of OLED. For example, Zhou et al. reported in xe2x80x9cVery-Low-Operating-Voltage Organic Light-Emitting Diodes Using a p-Doped Amorphous Hole Injection Layerxe2x80x9d, Applied Physics Letters, 78, 410 (2001) that high luminance efficiency and low drive voltage can be achieved in an OLED having an HIL comprising tetrafluoro-tetracyano-quinodimethane (F4-TCNQ) doped 4,4xe2x80x2,4xe2x80x3-tris(N,N-diphenyl-amino)triphenylamine (TDATA) in contact with both an anode and an HTL.
Using the aforementioned methods can effectively enhance the luminous efficiency of an OLED. However, multiple HTL structures cannot substantially improve the lifetime of the device. Instead, it usually shortens the lifetime of the device. Shown in FIGS. 1B and 2B are the schematic electron energy band diagrams of FIGS. 1A and 2A, respectively. As is known, the most commonly used anode, indium tin oxide (ITO), can have a work-function of about 5.0 eV with some proper surface treatments, and a TPD layer has an ionization potential (Ip) of about 5.6 eV. When a single TPD layer is used as an HTL in adjacent to ITO in an OLED, it creates an energy barrier of higher than 0.6 eV for hole-injection at the interface of ITO/TPD, and this high energy barrier can cause a fast interface damage during operation, resulting in short operational lifetime. In Shirota""s paper, an m-TDATA layer is used as another HTL between ITO and the TPD layer. Since m-TDATA layer has an Ip of about 5.1 eV, it forms an energy barrier of about 0.1 eV at the interface of ITO/m-TDATA. This low hole-injection barrier will not easily cause interface damage during operation. Therefore, the half-brightness lifetime of the OLED using m-TDATA/TPD layers as dual HTLs increases from 150 hrs to 300 hrs with an initial luminance of 300 cd/m2. Moreover, another energy barrier of about 0.5 eV is formed at the interface of m-TDATA/TPD. Although this barrier accumulates holes and slow down the transport of holes into the LEL resulting in high luminous efficiency, this barrier is also limits further improvement of device lifetime. A conventional OLED, having an N,Nxe2x80x2-bis(1-naphthyl)-N,Nxe2x80x2-diphenyl-1,1xe2x80x2-biphenyl-4,4xe2x80x2-diamine (NPB) layer as a single HTL and a tris(8-hydroxyquinoline)aluminum (Alq) layer as an ETL, usually has a half-brightness life time longer than 5,000 hrs with an initial luminance of 300 cd/m2. The substantial increase in lifetime is mainly due to the fact that NPB has an Ip of about 5.4 eV, and it forms a lower energy barrier at the interface of ITO/NPB compared to ITO/TPD, as well as due to the higher glass transition temperature of NPB than that of TPD. When NPB HTL is replaced by dual HTLs, the lifetime of the device can actually be reduced. If more than two HTLs are used in OLED and if the hole-injection barriers are not low enough to reduce interface damage at each interface in the HTR, lifetime of the OLED may not be improved. Moreover, F4-TCNQ is not thermally stable in its host layer, and it can diffuse from its host layer into the LEL to quench the luminance and to shorten the device lifetime. Furthermore, the fabrication of multiple HTLs or doped HTL (or doped HIL) needs more material sources in an evaporation chamber and needs longer device fabrication time, especially when more than two HTLs are fabricated.
It is therefore an object of the present invention to improve the luminous efficiency of an OLED.
It is another object of the present invention to simplify the layer structure of an OLED.
It is yet another object of the present invention to reduce the number of the materials sources needed for the fabrication of an OLED.
These objects are achieved by an organic electroluminescent device comprising:
a) an anode;
b) a hole-transporting region disposed over the anode; wherein the hole-transporting region contains at least one hole-transporting material;
c) a metal sub-layer disposed within the hole-transporting region; wherein the metal sub-layer contains at least one metal selected from group 4 through group 16 of the Periodic Table of Elements and the selected metal has a work-function higher than 4.0 eV;
c) a light-emitting layer formed in contact with the hole-transporting region for producing light in response to hole-electron recombination;
d) an electron-transporting layer disposed over the light-emitting layer; and
e) a cathode disposed over the electron-transporting layer.
The present invention makes use of a metal sub-layer in the HTR where the metal sub-layer contains a metal having a work-function higher than 4.0 eV. By this arrangement, an OLED can have a higher luminous efficiency compared to conventional OLEDs without compromising the operational lifetime. It also needs fewer material sources to fabricate this OLED compared to the fabrication of the OLED having multilayered HTLs. It is also unexpectedly found that this metal sub-layer will not cause a metal diffusion problem in the device.